1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system in which a latent image is formed on a photosensitive medium by scanning a laser beam, and the latent image is developed with a toner, so as to form an image.
2. Description of the Related Art
In printers and copying machines, a digital electrophotographic system is adopted widely as a method capable of providing high speed and high image quality. In this system, the optical scanning on a photosensitive medium is effected by using a light beam, and in order to reproduce gradations of the image, pulse-width modulation exposure is often carried out using an analog screen generator or the like (e.g., refer to Japanese Patent Unexamined Publication No. Hei. 1-280965).
In these printers and copying machines, image formation is effected by fixing both the spot diameter of the light beam and the number of lines in regions ranging from low-density portions to high-density portions. For this reason, the contrast of an exposure profile in a low-density portion declines and becomes analog-like. Further, since the amount of exposure itself is small, there have been problems in that the reproducibility of dots and lines deteriorates, and that the stability of gradation and color reproduction with respect to the environment deteriorates. These problems are particularly noticeable in cases where the following photosensitive materials, which are generally used as photosensitive materials, are used: a photosensitive material having an Se-based, an amorphous photosensitive layer exhibiting an attenuation of the potential approximately analogous to an amount of incident light, and a function-separated type photosensitive material comprised of a charge-generating layer and a charge-transporting layer and using an organic semiconductor.
With respect to the above-described problem, there has been proposed a system for stabilizing various elements, including the stabilization of the amount of the light beam and the stabilization of a toner concentration in a developing apparatus, and a system called process control in which the stability of gradation and color reproduction with respect to the environment is increased by measuring the temperature and humidity and the toner concentration in the developing apparatus and by controlling development bias and transfer current values (e.g., refer to Japanese Patent Unexamined Publication Nos. Hei. 4-37882 and Hei. 4-36776). However, these systems have a drawback in that a high-precision sensor and a control mechanism are required, with the result that the systems become complex and expensive.
With respect to the above-described problems of the photosensitive materials, the present inventors proposed an invention entitled "Image Forming Apparatus" so as to overcome the above-described problems. This is a high-image-quality color image forming apparatus which excels in gradation characteristics and can be realized with a compact size and light weight, wherein a photosensitive material in which attenuation of the potential occurs quickly on input thereto of a certain amount of light is used as a photosensitive material, and a substantially pulse-like electrostatic latent image is formed on the photosensitive material by effecting pulse modulation by setting the beam spot diameter in the main scanning direction to a size 0.5 times or less the pixel pitch in the main scanning direction as an exposure means. With this image forming apparatus, even if a nonmagnetic one-component developing system is used in which the dynamic range of the contrast potential in development is narrow, it is possible to faithfully develop an electrostatic latent image, and image formation based on a stable area-modulation method becomes possible. However, there have been cases where a reproduction starting point (an image signal value whereby development is started) particularly at a low-density portion becomes unstable due to the effect of changes in such as the spot diameter of the light beam and the light emission intensity of the light beam caused by changes in the environment.
In addition, systems have been proposed in which the spot diameter of the light beam and the light emission intensity of the light beam are made variable, a decline in the contrast of an exposure profile is suppressed, and the reproducibility of dots and lines is increased (e.g., Japanese Patent Unexamined Publication Nos. Hei. 4-13163, Hei. 4-97374, and Hei. 4-94261). However, these systems have a drawback in that they require a control mechanism for making variable the spot diameter of the light beam and the light emission intensity, with the result that the systems become complex and expensive.